Presently, solid-state devices are limited in their ability to deliver power at Terahertz (THz) frequencies. Device that exhibit good gain at high frequency (0.5-1 THz), such as Indium Phosphide (InP) Heterojunction Bipolar Transistors (HBTs) and Indium Aluminum Arsenide (InAlAs) Metamorphic High Electron Mobility Transistors (mHEMTs), sacrifice operating voltage to achieve this, and therefore run at very low voltage (<2 V) and low power. Conventional Gallium Nitride (GaN) devices can deliver high operating voltage (5-15 V) and therefore high power, but are restricted to lower frequencies (<0.4 THz) due to low gain. Conventionally, to achieve high Field Effect Transistor (FET) gain at high frequency, device designers will scale the device gate length (Lg) to very small values to reduce gate-source capacitance and increase transconductance. In addition, the gate-source and gate-drain dimensions are also made small to minimize parasitic access resistances. Both of these approaches have significant drawbacks. A small Lg is susceptible to short channel effects, which can severely limit the three-terminal operating voltage. Small gate-source and gate-drain spacing also reduces operating voltage through a reduction in the two-terminal breakdown voltage. Both techniques will reduce yield by making the device much harder to manufacture.
Certain heterostructure materials, such as Aluminum Gallium Nitride (AlGaN) and GaN, create an electron well (i.e., a sheet of electrons) at the interface between the two dissimilar materials resulting from the piezoelectric effect and spontaneous polarization effect therebetween. The resulting sheet of electrons that forms at this interface is typically referred to as a Two-Dimensional Electron Gas (“2DEG”) channel. Equally applicable is a superlattice structure having a plurality of two-dimensional hole gas (2DHG) channels. Both types of structures can be referred to as “2D×G channel(s)” devices. FETs that operate by generating and controlling the electrons in the 2D×G channel are conventionally referred to as high electron mobility transistors (“HEMTs”).
By stacking a plurality of these two-material heterostructures, and with the addition of appropriate doping in the layers to maintain the presence of the 2D×G channels when stacking a plurality of heterostructure layers, the electron sheets are able to act in parallel, allowing for greater current flow through the superlattice device. When this type of FET is “on”, the superlattice device has a lower on-resistance, relative to a single heterostructure-layer device, because the multiple 2DEG channels allow a proportionally higher current to flow between the source and drain, resulting in an overall reduction in on-resistance. This type of structure has been well suited for providing an ultra low channel resistance high frequency switch. However, they are not as ideally suited for forming highly linear amplifiers.